Information can be recorded by exposure of a portion of an optical recording medium to a recording light beam thereby changing the local optical properties of the exposed portion. The simplest such recording medium is a layer of a light absorptive material overlying a substrate wherein information is recorded by locally melting or ablating the absorptive layer to form a pit therein. The presence of the pit results in a change in the local transmission and/or the reflectivity of the recording medium, one of which is detected during readout of the information.
Spong, in U.S. Pat. No. 4,097,895, issued June 27, 1978 and entitled "MULTILAYERED OPTICAL RECORD", disclosed an optical recording medium which comprises a light reflective layer coated with a light absorptive layer, wherein the thickness of the absorptive layer is chosen so that the reflectivity of the recording medium is reduced. Bell, in U.S. Pat. No. 4,216,501 issued Aug. 5, 1980 and entitled "OPTICAL ANTIREFLECTIVE INFORMATION RECORD", has disclosed a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and absorptive layers of the Spong optical recording medium. This medium permits the use of a broader class of materials and a lower reflectivity recording medium or information record than that provided by the recording medium disclosed by Spong.
Spong et al, in U.S. patent application Ser. No. 254,649 filed Apr. 16, 1981 which is a continuation-in-part of in U.S. patent application Ser. No. 174,844 filed Aug. 4, 1980 now abandoned and entitled "INFORMATION RECORD AND A METHOD OF REVERSIBLY RECORDING AND ERASING INFORMATION THEREON", which is incorporated herein by reference, have disclosed an information record in which information can be recorded, erased and re-recorded and which includes a capping layer overlying the absorptive layer. The capping layer inhibits, up to a maximum power, irreversible recording such as the formation of a pit in the absorptive layer, upon exposure to an information recording or information erasing light beam.
It would be desireable to have a recording medium in which the range of recording beam powers which produce a reversible recording is larger than that known heretofore.